Bonded wafer structure having cavities with low pressure and method for forming

ABSTRACT

A multi-wafer structure is formed by forming a cavity in a cap wafer and forming a first seal material around the cavity. A collapsible standoff structure is formed around the cavity. A movable mass is formed in a device wafer. A second seal material is formed around the movable mass. The first seal material and the second seal material are of materials that are able to form a eutectic bond at a eutectic temperature. The cap wafer and the device wafer are arranged so that the first and second seals are aligned but separated by the collapsible standoff structure. Gas is evacuated from the cavity at a temperature above the eutectic temperature using a low pressure. The temperature is lowered, the cap and device wafer are pressed together, and the temperature is raised above the eutectic temperature to form a eutectic bond with the first and second seal materials.

BACKGROUND

1. Field

This disclosure relates generally to semiconductor processing, and morespecifically, to a bonded wafer structure having cavities with lowpressure.

2. Related Art

During wafer bonding, two wafers are typically stacked and aligned priorto bonding the wafer pair. In order to efficiently outgas the wafers,the wafers are separated by removable spacers, such as with metal tabswhich are part of the wafer handling equipment (e.g. pivoting metal tabsof a wafer chuck). After the outgassing, the wafers are brought togetherduring wafer bond in which the removable spacers are removed. However,the wafers frequently slip or move when the removable spacers areremoved, thus compromising the alignment of wafers. This is especiallyproblematic when bonding seals on each wafer must be aligned to createappropriate vacuum or low pressure cavities, such as formicro-electro-mechanical system (MEMS) devices. If the wafers slip andmisalign during removal of the removable spacers, the appropriate lowlevel of pressure cannot be maintained in the cavities which may resultin reduced device performance or reduced device longevity. Therefore, aneed exists for improved wafer bonding which allows for wafer separationduring outgassing without resulting in misalignment during bonding.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and is notlimited by the accompanying figures, in which like references indicatesimilar elements. Elements in the figures are illustrated for simplicityand clarity and have not necessarily been drawn to scale.

FIGS. 1-10 illustrate, in cross sectional form, a wafer pair at variousstages of processing in accordance with one embodiment of the presentinvention.

FIG. 11 illustrates, in flow diagram form, a method for forming a bondedwafer structure in accordance with one embodiment of the presentinvention.

FIGS. 12-18 illustrate, in cross sectional form, a wafer pair at variousstages of processing in accordance with one embodiment of the presentinvention.

FIGS. 19-22 illustrates, in cross sectional form, a wafer pair atvarious stages of processing in accordance with one embodiment of thepresent invention.

DETAILED DESCRIPTION

A wafer pair to be bonded to form a bonded wafer structure includes acap wafer and a device wafer. The cap wafer includes a plurality ofcavities, each surrounded by a first seal material, and the device waferincludes a plurality of devices, each surrounded by a second sealmaterial. The first and second seal materials are materials that arecapable of forming a eutectic bond at a eutectic temperature. The capand device wafers are stacked and aligned using collapsible standoffstructures such that the first seal material around each cavity alignswith the second seal material around a corresponding device. Whilestacked with the collapsible standoff structures in a bond chamber,outgassing is performed at a temperature above the eutectic temperatureto sufficiently remove trapped gasses from the wafer pair and establisha low pressure in the bond chamber. The temperature is then reduced tobelow the eutectic temperature, while the stacked wafer pair remainsseparated by the collapsible structures. A force is then applied tobring the cap and device wafer into contact by collapsing thecollapsible structures. This results in the first seal material and thesecond seal material of the cap wafer and device wafer, respectively,coming into contact. The temperature in the bond chamber is then rampedto the eutectic temperature such that the first and second sealmaterials form a eutectic bond. In this manner, a eutectic bondsurrounds each cavity/device pair of the bonded wafers, thus sealingeach device in a cavity with low pressure. Note that since thecollapsible standoff structures are not removed but are simply collapsedwhen force is applied to bring the cap and device wafer into contact,the cap and device wafers do not slip and misalign during the bondingprocess. With the effective formation of a cavity with low pressure foreach device, longevity and performance of each device may be improved.

FIG. 1 illustrates, in cross sectional form, a portion of a cap wafer 10in accordance with one embodiment of the present invention. Cap wafer 10includes a substrate 12, a seal layer 14 over substrate 12, and aninsulating layer 16 over seal layer 14. Openings 18 and 20 are formed ininsulating layer 16. Substrate 12 can be any semiconductor material orcombinations of materials, such as gallium arsenide, silicon germanium,silicon-on-insulator (SOI), silicon, monocrystalline silicon, the like,and combinations of the above. In one embodiment, insulating layer 16 isan oxide layer and may be formed using tetraethylorthosilicate (TEOS).In FIG. 1, the illustrated portion of cap wafer 10 corresponds to onecavity location of cap wafer 10. Each cavity location of cap wafer 10corresponds to a die location of a device wafer which will be bondedwith cap wafer 10, and which will be described in further detail below.Note that openings similar to openings 18 and 20 can be formed for everycavity location of cap wafer 10. Seal layer 14 includes a first sealmaterial, such as, for example, germanium.

FIG. 2 illustrates, in cross sectional form, cap wafer 10 at asubsequent stage in processing in which a polysilicon layer 22 is formedover insulating layer 16, and within openings 18 and 20. In oneembodiment, polysilicon layer 22 is thinner than insulating layer 16,and does not fill openings 18 and 20.

FIG. 3 illustrates, in cross sectional form, cap wafer 10 at asubsequent stage in processing. Polysilicon layer 22 is patterned toremove portions of polysilicon layer 22 between the openings ininsulating layer 16. Portions of layer 22 remaining in the openings formcollapsible standoff structures. Therefore, collapsible standoffstructure 24, formed from layer 22, remains in opening 18 andcollapsible standoff structure 26, formed from layer 22, remains inopening 20. Each collapsible standoff structure is in contact withunderlying seal layer 14.

FIG. 4 illustrates, in cross section form, cap wafer 10 at a subsequentstage in processing. An opening 28 is formed between openings 18 and 20.Opening 28 extends through insulating layer 16 and seal layer 14, andextends into substrate 12. An opening, such as opening 28, is formed tooverlie locations on the device wafers where a low pressure cavity, e.g.vacuum, is desired. In this manner, as illustrated in FIG. 4, aremaining portion of seal layer 14 forms a seal ring around opening 28.

FIG. 5 illustrates, in cross section form, cap wafer 10 at a subsequentstage in processing in which cavities are formed within the openings,such as opening 28. Therefore, a cavity 30 and a cavity 32 are formedwithin opening 28 which extend deeper into substrate 12. A remainingportion of substrate 12 between cavities 30 and 32 corresponds to anover-travel stop 34 of a MEMS device, as will be described in moredetail below. (Note that opening 28 may be referred to as a cavity, andcavities 30 and 32 may be referred to as sub-cavities.)

FIG. 6 illustrates, in cross section form, cap wafer 10 at a subsequentstage in processing. Insulating layer 16 is removed from cap wafer 10resulting in collapsible standoff structures 24 and 26 no longer beingsupported by an insulating layer. As will be described below, theresulting standoff structures, which are in contact with seal layer 14,are sufficiently strong to not collapse when inverted and stacked uponanother wafer. However, they are sufficiently fragile such that with theapplication of appropriate force, they do collapse.

FIG. 7 illustrates, in cross sectional form, a portion of a device wafer40 in accordance with one embodiment of the present invention. Devicewafer 40 includes a plurality of die in which each die includes adevice, such as a MEMS device 56. Device wafer 40 includes a substrate42, a dielectric layer 44 over substrate 42, and a silicon layer 46having an opening which exposes dielectric layer 44. MEMS device 56 islocated within the opening having a routing layer 48 in contact withdielectric layer 44, a spring portion 50 over routing layer 48, and amovable mass 52 over spring portion 50. Each of portions 48, 50, and 52may be made and patterned as silicon layer 46 is formed. That is, layer46 may include a number of layers which achieves the resulting thicknessof layer 46 and which may be used to form each portion of the MEMSdevice. MEMS device 56 may be any type of MEMS device such as aresonator, a gyroscope, accelerometer, etc. Therefore, the portions ofMEMS device 56 may be formed as needed. In one embodiment, spring 50 maybe a pivot or an anchor for movable mass 52. In another embodiment,movable mass 52 may also function as a sense capacitance structure or asan electrical interconnect. A seal ring 54 over polysilicon layer 46surrounds MEMS device 56. In one embodiment, seal ring 54 is patternedfrom a seal layer formed over polysilicon layer 46 and includes a secondseal material, such as aluminum.

FIGS. 8-10 will be described in further detail in combination withmethod 80 illustrated in flow diagram form in FIG. 11. In FIG. 11,method 80 begins with block 82 in which the seal aluminum and the sealgermanium of the cap and device wafers, respectively, are aligned toform an aligned wafer pair with standoffs in contact with the cap anddevice wafers of the wafer pair. Referring to FIG. 8, FIG. 8illustrates, in cross sectional form, cap wafer 10 (from FIG. 6)inverted and stacked onto device wafer 40. Cap wafer 10 and device wafer40 are aligned such that cavities 30 and 32 are aligned over device 56and seal ring 14 (which may be referred to as the seal germanium) isaligned over seal ring 54 (which may be referred to as the sealaluminum). In one embodiment, wafers 40 and 10 are first stacked andsubsequently aligned. Collapsible standoff structures 26 and 24 contactboth seal ring 14 of cap wafer 10 and seal ring 54 of device wafer 40and support cap wafer 10 on top of device wafer 40. Collapsible standoffstructures ensure that cap wafer 10 and device wafer 40 remain spacedapart by a distance equivalent to the height of the standoff structures.They do not collapse under the weight of one wafer stacked on top. Thealigned wafer pair is then placed into a bond chamber, as indicated inblock 84 of method 80 in FIG. 11. Note that cap wafer 10 is in contactwith a down force mechanism 58, but no down force is yet applied.Although, as illustrated, cap wafer 10 is represented as being incontact with down force mechanism 58, the positions of device wafer 40and cap wafer 10 may be reversed such that device wafer 40 is in contactwith down force mechanism 58 instead.

Once in the bond chamber, method 80 continues to block 86 in which avacuum pump is used to remove trapped gasses from the bond chamber andthe wafer pair at a temperature that is above ambient temperature. Thiscommences the outgassing of trapped gasses from the wafer pair. Method80 continues to block 88 in which the temperature in the bond chamber isramped to a temperature that is greater than the eutectic temperature ofAlGe to further outgas the wafer pair and to establish reduced pressurein the bond chamber. By performing the outgassing at a high temperature(greater than the eutectic temperature of the combination of the twoseal ring materials), improved outgassing is achieved resulting in animproved low pressure environment within the bond chamber. Standoffstructures 26 and 24 prevent seal rings 14 and 54 from prematurelybonding prior to completing the outgassing. Since the outgassingtemperature is at a temperature greater than the eutectic temperature,seal rings 14 and 54 need to remain spaced apart during outgassing. Notethat the outgassing evacuates gas from cavities 32, 30, and in thematerials surrounding MEMS device 56 in preparation for creating a lowpressure cavity for MEMS device 56.

Referring still to FIG. 11, method 80 continues to block 90 in which thetemperature is ramped back down to a temperature that is below theeutectic temperature of AlGe. Once the temperature is below the eutectictemperature, method 80 proceeds to block 92 in which a force is appliedto bring the cap and device wafers into contact with each other bycollapsing the standoff structures.

FIG. 9 illustrates, in cross sectional form, application of a down force60 using down force mechanism 58 which results in collapsing standoffstructures 26 and 24. Therefore, cap wafer 10 and device wafer 40 arepressed together with sufficient force to bring seal ring 14 intocontact with seal ring 54. Method 80 of FIG. 11 then proceeds to block94 in which the temperature is ramped to the eutectic temperature ofAlGe to bond the wafer pair in which the bonded wafer pair includes lowpressure cavities.

FIG. 10 illustrates, in cross section form, the result of bringing capwafer 10 into contact with device wafer 40 and raising the temperatureto the eutectic temperature of AlGe such that seal ring 14 and seal ring54 form a eutectic bond, resulting in a eutectic seal 68. Eutectic seal68 surrounds a cavity which is formed by cavities 30 and 32 and thespaces around MEMS device 56. Note also that travel stop 34 can operateas a travel stop for moveable mass 52. Since eutectic seal 68 was formedin the bond chamber having reduced pressure, cavity 70 has low pressure.For example, cavity 70 may be a vacuum.

In the formation of eutectic seal 68 in which the seal material of sealring 14 is germanium and the seal material of seal ring 54 is aluminum,eutectic seal 68 results in having germanium portions 62 and aluminumportions 64. Above the eutectic temperature, the sealing alloy of thetwo seal materials is liquid so a good quality seal with no empty spaces(i.e. voids) is formed. Portions of the collapsible structure aresurrounded by the alloyed materials during this process. The resultingeutectic material and ratios of Al and Ge may be controlled by thethicknesses of seal ring 14 and seal ring 54. Also, while the examplesof FIGS. 1-11 have been described in reference to aluminum and germaniumas the two seal materials bonded together, other materials may be usedas the seal ring on the device wafer and seal ring on the cap wafer toform different eutectic and non-eutectic bonds, other than eutecticAlGe. Therefore, eutectic seal 68 may also be referred to as a sealingalloy ring. Also, due to the collapse of collapsible standoff structures24 and 26, eutectic seal 68 may include polysilicon fragments 66 whichbroke off from the collapsible standoffs structures. Polysiliconfragments 66 may therefore be referred to as a collapsed standoffstructure which is located within eutectic seal 68. The design ofcollapsible standoff structures 24 and 26 may also be such thatdeformation during the bonding process does not cause plasticdeformation or fracture, but rather elastically deforms the structureinto a flat form that then integrates with the newly formed sealingalloy and is frozen in the flat state.

In the embodiments of FIGS. 1-10, note that the standoff structures wereformed in the region in which seal rings 14 and 54 are aligned to formsealing alloy ring 68 which surrounds cavity 70. That is, upon stackingthe wafers, the standoff structures are between seal rings 14 and 54, inwhich seal rings 14 and 54 react to become the eutectic seal ring whichseals in low pressure cavity 70. Since it is possible that a fragment ofa standoff structure upon being collapsed may enter into cavity 70, inan alternate embodiment, the standoff structures may be formed outsideof alignment region of the seal rings such as seal rings 14 and 54.

FIGS. 12-18 illustrate, in cross section form, formation of a bondedwafer pair with the use of collapsible structures formed outside of theseal rings. FIG. 12 illustrates, in cross sectional form, a portion of acap wafer 100 in accordance with one embodiment of the presentinvention. Cap wafer 100 includes a substrate 120, a seal layer 140 oversubstrate 120, and an insulating layer 142 over seal layer 140. Openings144 and 146 are formed in insulating layer 142. A layer 148 is formedover insulating layer 142, and within openings 144 and 146. In oneembodiment, layer 148 is thinner than insulating layer 142 and does notfill openings 144 and 146. Substrate 120 can be any semiconductormaterial or combinations of materials, such as gallium arsenide, silicongermanium, silicon-on-insulator (SOI), silicon, monocrystalline silicon,the like, and combinations of the above. In one embodiment, insulatinglayer 142 is an oxide layer and may be formed usingtetraethylorthosilicate (TEOS). In FIG. 12, the illustrated portion ofcap wafer 100 corresponds to one cavity location of cap wafer 100. Eachcavity location of cap wafer 100 corresponds to a die location of adevice wafer which will be bonded with cap wafer 100, and which will bedescribed in further detail below. Note that openings similar toopenings 144 and 146 can be formed for every cavity location of capwafer 10. Seal layer 140 includes a first seal material, such as, forexample, germanium or aluminum. Layer 148 may be a polysilicon layer ormay be of a different material, such as aluminum. For example, in thecase that seal layer 140 is germanium, layer 148 may be a polysiliconlayer, and in the case that seal layer 140 is aluminum, layer 148 may bean aluminum layer.

FIG. 13 illustrates, in cross sectional form, cap wafer 100 at asubsequent stage in processing. Layer 148 is patterned to removeportions of layer 148 between the openings in insulating layer 142.Portions of layer 148 remaining in the openings form collapsiblestandoff structures. Therefore, collapsible standoff structure 150,formed from layer 148, remains in opening 144 and collapsible standoffstructure 152, formed from layer 148, remains in opening 146. Eachcollapsible standoff structure is in contact with underlying seal layer140. Depending on seal material of seal layer 140, collapsible standoffstructures 150 and 152 may either be polysilicon or aluminum.

FIG. 14 illustrates, in cross section form, cap wafer 100 at asubsequent stage in processing. Openings 156, 158, and 160 are formed ininsulating layer 142, and they each extend through insulating layer 142and seal layer 140, and into substrate 120. Opening 158 is formedbetween openings 144 and 146, opening 156 is formed between opening 144and 158, and opening 160 is formed between opening 158 and 146. Theseopenings define a portion 159 of seal layer 140 which surrounds cavity158, and may therefore be referred to as seal ring 159. As will bedescribed below, seal ring 159 will form the eutectic seal ring whichencloses the final low pressure cavity containing the MEMS device. Notethat collapsible standoff structures 150 and 152 are spaced apart fromseal ring 159 by openings 156 and 160. Openings such as openings 156,158, and 160 are formed in each die location of cap wafer 100.

FIG. 15 illustrates, in cross section form, cap wafer 100 at asubsequent stage in processing in which cavities are formed withinopening 158. Therefore, a cavity 162 and a cavity 164 are formed withinopening 158 which extend deeper into substrate 120 than openings 156,158, and 160. A remaining portion of substrate 120 between cavities 162and 164 correspond to a travel stop 163 of a MEMS device, as will bedescribed in more detail below.

FIG. 16 illustrates, in cross section form, cap wafer 100 at asubsequent stage in processing. Insulating layer 142 is removed from capwafer 100 resulting in collapsible standoff structures 150 and 152 nolonger being supported by an insulating layer. As with collapsiblestandoff structures 24 and 26, resulting standoff structures 150 and 152are sufficiently strong to not collapse when inverted and stacked uponanother wafer. However, they are sufficiently fragile such that with theapplication of appropriate force, they do collapse.

FIG. 17 illustrates, in cross sectional form, cap wafer 100 placed overa portion of a device wafer 200 in accordance with one embodiment of thepresent invention. Device wafer 200 includes a plurality of die in whicheach die includes a device, such as a MEMS device 217. Device wafer 200includes a substrate 204, a dielectric layer 206 over substrate 204, anda silicon layer 208 having an opening which exposes dielectric layer206. MEMS device 217 is located within the opening having a routinglayer 212 in contact with dielectric layer 206, a spring portion 214over routing layer 212, and a movable mass 216 over spring portion 214.Note that the descriptions of substrate 204, dielectric layer 206,silicon layer 208, MEMS device 217, routing layer 212, spring portion214, and movable mass 216 are analogous to the descriptions of substrate42, dielectric layer 44, silicon layer 46, MEMS device 56, routing layer48, and a spring portion 50, respectively, of device wafer 40 describedabove. A seal ring 210 over polysilicon layer 208 surrounds MEMS device217. In one embodiment, seal ring 210 is patterned from a seal layerformed over polysilicon layer 208 and includes a second seal material.This second seal material is a seal material which is capable of forminga eutectic bond with the first seal material of seal ring 159.Therefore, if seal ring 159 is germanium, the second seal material maybe aluminum, and if seal ring 159 is aluminum, the second seal materialmay be germanium.

As illustrated in FIG. 17, cap wafer 100 and device wafer 200 arealigned to form an aligned wafer pair with standoffs in contact with thecap and device wafers of the wafer pair. Cap wafer 100 is inverted andstacked onto device wafer 200. Cap wafer 100 and device wafer 200 arealigned such that cavities 164 and 162 are aligned over device 217 andseal ring 159 is aligned over seal ring 210. In one embodiment, wafers200 and 100 are first stacked and subsequently aligned. Collapsiblestandoff structures 152 and 150 contact polysilicon layer 208 of devicewafer 200 and a remaining portion of seal layer 140 of cap wafer 100.Collapsible standoff structures 152 and 150 are spaced apart from sealrings 159 and 210 and thus are outside a region where seal rings 159 and310 are aligned. Collapsible standoff structures 152 and 150 ensure thatcap wafer 100 and device wafer 200 remain spaced apart by a distanceequivalent to the height of the standoff structures. They do notcollapse under the weight of one wafer stacked on top. The aligned waferpair is then placed into a bond chamber. Note that the temperaturesdescribed above in method 80 of FIG. 11 also apply to the wafer pair ofFIGS. 17 and 18.

Once in the bond chamber, a vacuum pump is used to remove trapped gassesfrom the bond chamber and the wafer pair at a temperature that is aboveambient temperature. This commences the outgassing of trapped gassesfrom the wafer pair. The temperature in the bond chamber is then rampedto a temperature that is greater than the eutectic temperature of AlGeto further outgas the wafer pair and to establish reduced pressure inthe bond chamber. By performing the outgassing at a high temperature(greater than the eutectic temperature of the combination of the twoseal ring materials), improved outgassing is achieved resulting in animproved low pressure environment within the bond chamber. Standoffstructures 152 and 150 prevent seal rings 159 and 210 from prematurelybonding prior to completing the outgassing. Since the outgassingtemperature is at a temperature greater than the eutectic temperature,seal rings 159 and 210 need to remain spaced apart during outgassing.Note that the outgassing evacuates gas from cavities 164 and 162 inpreparation for creating a low pressure cavity for MEMS device 217.

After a sufficient amount of time for the outgassing, the temperature isramped back down to a temperature that is below the eutectic temperatureof AlGe. Once the temperature is below the eutectic temperature, a forceis applied to bring the cap and device wafers into contact with eachother by collapsing the standoff structures. If the collapsible standoffstructures are polysilicon, they will be collapsed into fragments, asdescribe above in reference to standoff structures 24 and 26. If thecollapsible standoff structures are aluminum, they would be bent anddeformed, allowing the wafers to be brought together. By collapsiblestandoff structures 152 and 150 collapsing with the application of adown force, seal ring 159 is brought into contact with seal ring 210.Once the seal rings are brought into contact, the temperature is rampedto the eutectic temperature of AlGe to bond the wafer pair in which thebonded wafer pair includes low pressure cavities.

FIG. 18 illustrates, in cross section form, the result of bringing capwafer 100 into contact with device wafer 200 and raising the temperatureto the eutectic temperature of AlGe such that seal ring 159 and sealring 210 form a eutectic bond, resulting in a eutectic seal 220.Eutectic seal 220 surrounds a cavity 222 which is formed by cavities 162and 164 and the spaces around MEMS device 217. Note also that travelstop 163 can operate as a travel stop for moveable mass 216. Sinceeutectic seal 220 was formed in the bond chamber having reducedpressure, cavity 222 has low pressure. For example, cavity 222 may be avacuum. FIG. 18 illustrates the example in which collapsible standoffstructures 152 and 150 are aluminum and get deformed between seal layer140 and polysilicon layer 208. In the example in which collapsiblestandoff structures 152 and 150 are polysilicon, a portion of thestandoff structures may remain between seal layer 140 and polysiliconlayer 208 and fragments may also remain present. However, thesefragments would not be located within eutectic seal 220 nor would theybe able to enter cavity 222. Regardless of the material used for thestandoff structures, due to their location with respect to eutectic bond220, eutectic bond 220 would be substantially free from portions ofcollapsible standoff structures 152 and 150.

As with eutectic seal 68, the resulting eutectic material and ratios ofAl and Ge may be controlled by the thicknesses of seal ring 159 and sealring 210. Also, while the examples of FIGS. 12-18 have been described inreference to aluminum and germanium as the two seal materials bondedtogether, other materials may be used as the seal ring on the devicewafer and seal ring on the cap wafer to form different eutectic bonds,other than eutectic AlGe.

With MEMS devices 56 and 217 being in a low pressure cavity, theperformance and longevity of the devices may be improved as compared todevices in which a low pressure cavity is not properly formed. Forexample, not outgassing at a temperature that is greater than theeutectic temperature prior to wafer bonding or a misalignment of sealrings 14 and 54 or seal rings 159 and 210 upon raising the temperatureto the eutectic temperature for bonding may result in an increasedpressure within cavities 70 and 222 which would adversely affectperformance and longevity of the MEMS devices. Therefore, the abilityfor the standoff structures, such as standoff structures 24 and 26 orstandoff structures 150 and 152, to withstand a temperature greater thanthe eutectic temperature of the materials of the aligned seal ringsallows the wafer pair to maintain separation for improved outgassing atthe greater temperature. The temperature can be then be lowered again tobelow the eutectic temperature to allow the wafers to come into contactat the lower temperature prior to forming the eutectic bond.Furthermore, by collapsing under the force of the down pressuremechanism prior to raising the temperature to the eutectic temperature,the collapsible standoff structures allow for the alignment between sealrings 14 and 54 or between seal rings 159 and 210 to be maintained. Thatis, since the collapsible standoff structures collapse and need not beremoved, there is minimal risk of slipping or misalignment between thewafer pair. The improved alignment of the two wafers used to form thehermetic MEMS structure may also allow for improved electricalconnections in the case where the cap wafer also includes electricalconnections to the device wafer.

FIGS. 19-22 illustrate in cross section form, formation of a bondedwafer pair with the use of a collapsible structure formed by a post andcantilever aligned with each other. In one embodiment, the post isformed on the cap wafer and the cantilever is formed on the devicewafer. FIG. 19 illustrates a wafer pair including a cap wafer 300 and adevice wafer 302. Cap wafer 300 may be formed in a similar fashion tocap wafers 10 and 100. For example, cap wafer 300 includes a seal layer320 formed over a substrate in which openings are formed to define aseal layer portion 324 and a seal ring 322. These openings are similarto openings 156, 158, and 160. Further, additional openings may beformed to form cavities separate by a travel stop within the openinginside seal ring 322, similar to cavities 30 and 32 or cavities 162 and164. A polysilicon bond post 326 is formed over seal layer portion 324.Polysilicon bond post 326 may be formed by depositing and patterning apolysilicon layer over seal layer 320. In FIG. 19, cap wafer 300 isinverted and aligned over device wafer 302. Device wafer 302 is similarto devices wafers 40 and 202 described above. For example, device wafer302 includes a dielectric layer 309 over a substrate, a MEMS device 306within an opening in a polysilicon layer, and a seal ring 308 formedover the polysilicon layer and around MEMS device 306. MEMS device 306includes a routing layer 303, a spring portion 305 (which may be a pivotor anchor), and a movable mass 307. These may be similar to dielectriclayer 206 or 44, MEMS devices 56 or 217, and seal ring 54 or 210,respectively, which have been described above. Device wafer 302 alsoincludes a cantilever 304 outside of seal ring 308. Cantilever 304 is ata distance 310 above the substrate of device wafer 302.

As illustrated in FIG. 19, cap wafer 300 is inverted and aligned overdevice wafer 302 such that bond post 326 is aligned with cantilever 304and seal ring 322 is aligned with seal ring 308. Note that cap wafer 300and device wafer 302 have not yet been brought into contact. FIG. 21illustrates a cross section of the wafer pair of FIG. 19 taken from aperspective that is 90 degrees from the perspective of FIG. 19.Therefore, the length of cantilever 304 is visible. Cantilever 304 issupported by a layer 340 and a pivot 342. Cantilever 304, pivot 342, andlayer 340 may be formed at the same time as the moveable mass 307,spring portion 305, and routing layer 303 of MEMS device 306. Therefore,they are formed of the same materials as the corresponding portion ofMEMS device 306. Also, as with the embodiments described above, sealring 322 may include a first seal material, and seal ring 308 mayincludes a second seal material which is capable of forming a eutecticbond with the first seal material. For example, seal ring 322 may bealuminum and seal ring 308 may be germanium, or seal ring 322 may begermanium and seal ring 308 may be aluminum. Alternatively, other sealmaterials may be used to form a different eutectic bond other than anAlGe eutectic bond.

In a bond chamber, cap wafer 300 is aligned and stacked onto devicewafer 302. Upon being stacked, polysilicon post 326 rests uponcantilever 304 and maintains seal ring 322 spaced apart from seal ring308. The separation distance between the seal rings is determined by thethickness of polysilicon post 326 as measured from seal layer 324. Notethat pivot 304 is stiff enough so that it does not allow cantilever 304to tilt downward with just the weight of the cap wafer on top. That is,distance 310 is maintained, even with cap wafer 300 in contact withdevice wafer 302. Therefore, the combination of post 326 and cantilever304 form a collapsible standoff structure which maintain the separationbetween wafers upon being stacked and aligned in the bond chamber. Asdescribed above in the embodiments of FIGS. 1-17, the temperature israised to a temperature above the eutectic temperature of the alignedseal rings for improved outgassing. While the wafers maintain theirseparation, the temperature is again lowered to below the eutectictemperature.

FIG. 22 illustrates a cross sectional view of the wafer pair from thesame perspective as FIG. 21 after application of a force to bring sealrings 322 and 308 into physical contact. Under a down force, polysiliconpost 326 pushes down upon cantilever 304 with sufficient force todisplace cantilever 304 such that distance 310 is reduced to a distance328 between cantilever 304 and the substrate of device wafer 302.Therefore, by depressing cantilever 304, the aligned seal rings arebrought into contact while the temperature is still below the eutectictemperature. Once the seal rings are in contact, the temperature israised to the eutectic temperature.

FIG. 20 illustrates a cross section view of the wafer pair, from thesame perspective of FIG. 19, after formation of a eutectic bond betweenseal ring 322 and seal ring 308 to form a eutectic ring 332 surroundingcavity 330. Cavity 330 is a low pressure cavity surrounding MEMS device306, similar to low pressure cavities 70 and 222. As with collapsiblestandoff structures 24, 26, 50, and 52, collapsible standoff structures326 and 304 allow for alignment between seal rings 322 and 308 to bemaintained. That is, since the collapsible standoff structures collapsedue to pivot 342 and the force upon cantilever 304 and thus need not beremoved, there is minimal risk of slipping or misalignment between thewafer pair. Therefore, the collapsible standoff structure allows forproper formation of cavity 330 by maintaining wafer separation whileoutgassing at a temperature above the eutectic temperature and bymaintaining proper alignment for formation of the eutectic bond.Properly formed low pressure cavity 330 allows for improved performanceand longevity of the MEMS device.

Therefore, by now it should be appreciated how collapsible structuresare used to result in improved formation of low pressure cavities withina bonded wafer pair, which in turn may provide for improved performanceand longevity of MEMS devices within the low pressure cavities. Notethat the low pressure cavities may also include other devices which maybenefit from such low pressure cavities other than or in addition toMEMS devices. The collapsible standoff structures allow for outgassingto be performed at a temperature greater than the eutectic temperatureof the corresponding seal materials of the seal rings of the wafer pairby maintaining a separation between the wafer pair. This allows forreduced pressure within the bond chamber and improved outgassing. Afterthe temperature is lowered to below the eutectic temperature, a force isused to collapse the collapsible standoff structures to bring thecorresponding seal rings into contact. Since the collapsible standoffstructures collapse and are thus not removed, slipping or misalignmentof the wafer pair is reduced. Once the aligned seal rings are incontact, the temperature can again be raised to the eutectic temperatureto form a eutectic ring, resulting in a low pressure cavity between thewafer pair.

Because the apparatus implementing the present invention is, for themost part, composed of electronic components and circuits known to thoseskilled in the art, circuit details will not be explained in any greaterextent than that considered necessary as illustrated above, for theunderstanding and appreciation of the underlying concepts of the presentinvention and in order not to obfuscate or distract from the teachingsof the present invention.

Although the invention has been described with respect to specificconductivity types or polarity of potentials, skilled artisansappreciated that conductivity types and polarities of potentials may bereversed.

Moreover, the terms “front,” “back,” “top,” “bottom,” “over,” “under,”“above,” “below” and the like in the description and in the claims, ifany, are used for descriptive purposes and not necessarily fordescribing permanent relative positions. It is understood that the termsso used are interchangeable under appropriate circumstances such thatthe embodiments of the invention described herein are, for example,capable of operation in other orientations than those illustrated orotherwise described herein.

Although the invention is described herein with reference to specificembodiments, various modifications and changes can be made withoutdeparting from the scope of the present invention as set forth in theclaims below. For example, different types of MEMS devices may beincluded in each low pressure cavity having different structures thatthose examples illustrated in FIGS. 1-22. Accordingly, the specificationand figures are to be regarded in an illustrative rather than arestrictive sense, and all such modifications are intended to beincluded within the scope of the present invention. Any benefits,advantages, or solutions to problems that are described herein withregard to specific embodiments are not intended to be construed as acritical, required, or essential feature or element of any or all theclaims.

Furthermore, the terms “a” or “an,” as used herein, are defined as oneor more than one. Also, the use of introductory phrases such as “atleast one” and “one or more” in the claims should not be construed toimply that the introduction of another claim element by the indefinitearticles “a” or “an” limits any particular claim containing suchintroduced claim element to inventions containing only one such element,even when the same claim includes the introductory phrases “one or more”or “at least one” and indefinite articles such as “a” or “an.” The sameholds true for the use of definite articles.

Unless stated otherwise, terms such as “first” and “second” are used toarbitrarily distinguish between the elements such terms describe. Thus,these terms are not necessarily intended to indicate temporal or otherprioritization of such elements.

The following are various embodiments of the present invention.

In one embodiment, a method of forming a multi-wafer structure using capwafer and a device wafer each having a plurality of die includes forminga cavity in a die of the cap wafer; forming a first seal material aroundthe cavity; forming a collapsible standoff structure around the cavity;forming a movable mass in a die of the device wafer; forming a secondseal material around the movable mass that is alignable to the firstseal material, wherein the first seal material and the second sealmaterial are of materials that are able to form a eutectic bond at aeutectic temperature that is below the melting temperature of each ofthe first and second seal materials; arranging the cap wafer and thedevice wafer so that the first and second seals are aligned butseparated by the collapsible standoff structure; performing anevacuation of gas from the cavity at a temperature above the eutectictemperature using a low pressure; after sufficient time to provide theevacuation of gas and while maintaining the low pressure, reducing thetemperature so that the first and second seal materials are below theeutectic temperature; pressing the cap wafer and device wafer togetherwith sufficient force to cause the first and second seal materials to bein contact; and raising the temperature of the first and second sealmaterials above the eutectic temperature to form the eutectic bondbetween the first and second seal materials. In one aspect, the pressingthe cap wafer and device wafer together causes the collapsible standoffstructure to be crushed; and the raising the temperature results in atleast portions of the collapsible standoff structure being embedded inthe eutectic bond. In another aspect, the first seal material comprisesgermanium and the second seal material comprises aluminum. In anotheraspect, the cavity includes a first sub-cavity, a second sub-cavity, anda travel stop between the first sub-cavity and the second sub-cavity. Inanother aspect, the forming the collapsible standoff structure includesdepositing a first layer over the cap wafer having a first thickness;forming standoff openings in the first layer; forming a second layer,thinner than the first layer, in the standoff openings; removingportions of the second layer between the standoff openings to leaveportions of the second layer in the openings to form the collapsiblestandoff structure; and removing the first layer. In a further aspect,the first layer includes oxide and the second layer includespolysilicon. In another further aspect, the standoff openings are in theregion where the first and second seals will be aligned. In anotherfurther aspect, the standoff openings are spaced outside the regionwhere the first and second seals will be aligned. In yet a furtheraspect, the method further includes forming a spacer opening in the capwafer between the standoff openings and the region where the first andsecond seals will be aligned. In a further aspect, the eutectic bond issubstantially free from portions of the collapsible standoff structure.In another aspect of the above one embodiment, the forming thecollapsible standoff structure is further characterized as being furtherformed around the movable mass. In a further aspect, the forming thecollapsible standoff structure includes forming a post and a cantileverthat are aligned during the arranging the cap wafer and the devicewafer. In an even further aspect, the cantilever is formed in the devicewafer during the forming the movable mass. In an even further aspect,the post contacts the cantilever during the performing the evacuation ofthe gas and depresses the cantilever in response to the pressing the capwafer and device wafer together.

In another embodiment, a multi-wafer structure having a cap wafer and adevice wafer includes a cavity in a die of the cap wafer; a movable massin a die of the device wafer and in the cavity; a eutectic bondcontacting the cap wafer and the device wafer and sealing in a lowpressure in the cavity; and a collapsed standoff structure around thecavity and contacting the cap wafer and the device wafer, wherein thecollapsed standoff structure comprises one of a group consisting offractured pieces in the collapsed structure and a post depressing acantilever adjacent to the eutectic bond. In one aspect, the eutecticbond includes germanium and aluminum. In another aspect, the fracturedpieces include polysilicon. In another aspect, the cantilever is of thesame material is the movable mass. In another aspect, the structurefurther includes an opening in the device wafer adjacent to the eutecticbond on a side opposite that adjacent to the movable mass.

In yet another embodiment, a method of forming a MEMS device includesforming a cavity in a cap wafer; forming a movable mass in a devicewafer; forming a first material surrounding the cavity; forming a secondmaterial surrounding the movable mass; forming a collapsible standoff;aligning the cap wafer and the device wafer so that the movable mass isin the cavity and the cap wafer and device wafer are held in separationby the collapsible standoff; reducing the pressure in the cavity withthe aid of applying a heat greater than a eutectic temperature and ofthe first material and the second material; reducing the temperaturebelow the eutectic temperature; contacting the first material and thesecond material with a force sufficient to collapse the collapsiblestandoff; and applying a temperature greater than the eutectictemperature to form a eutectic bond with the first material and thesecond material to seal in a low pressure in the cavity with the movablemass.

What is claimed is:
 1. A method of forming a multi-wafer structure usingcap wafer and a device wafer each having a plurality of die, comprising:forming a cavity in a die of the cap wafer; forming a first sealmaterial around the cavity; forming a collapsible standoff structurearound the cavity; forming a movable mass in a die of the device wafer;forming a second seal material around the movable mass that is alignableto the first seal material, wherein the first seal material and thesecond seal material are of materials that are able to form a eutecticbond at a eutectic temperature that is below the melting temperature ofeach of the first and second seal materials; arranging the cap wafer andthe device wafer so that the first and second seals are aligned butseparated by the collapsible standoff structure; performing anevacuation of gas from the cavity at a temperature above the eutectictemperature using a low pressure; after sufficient time to provide theevacuation of gas and while maintaining the low pressure, reducing thetemperature so that the first and second seal materials are below theeutectic temperature; pressing the cap wafer and device wafer togetherwith sufficient force to cause the first and second seal materials to bein contact; and raising the temperature of the first and second sealmaterials above the eutectic temperature to form the eutectic bondbetween the first and second seal materials.
 2. The method of claim 1,wherein: the pressing the cap wafer and device wafer together causes thecollapsible standoff structure to be crushed; and the raising thetemperature results in at least portions of the collapsible standoffstructure being embedded in the eutectic bond.
 3. The method of claim 1,wherein the first seal material comprises germanium and the second sealmaterial comprises aluminum.
 4. The method of claim 1, wherein thecavity comprises a first sub-cavity, a second sub-cavity, and a travelstop between the first sub-cavity and the second sub-cavity.
 5. Themethod of claim 1, wherein the forming the collapsible standoffstructure comprises: depositing a first layer over the cap wafer havinga first thickness; forming standoff openings in the first layer; forminga second layer, thinner than the first layer, in the standoff openings;removing portions of the second layer between the standoff openings toleave portions of the second layer in the openings to form thecollapsible standoff structure; and removing the first layer.
 6. Themethod of claim 5, wherein the first layer comprises oxide and thesecond layer comprises polysilicon.
 7. The method of claim 5, wherein,the standoff openings are in the region where the first and second sealswill be aligned.
 8. The method of claim 5, wherein, the standoffopenings are spaced outside the region where the first and second sealswill be aligned.
 9. The method of claim 8, further comprising forming aspacer opening in the cap wafer between the standoff openings and theregion where the first and second seals will be aligned.
 10. The methodof claim 9, wherein, the eutectic bond is substantially free fromportions of the collapsible standoff structure.
 11. The method of claim1, wherein the forming the collapsible standoff structure is furthercharacterized as being further formed around the movable mass.
 12. Themethod of claim 11, wherein the forming the collapsible standoffstructure comprises forming a post and a cantilever that are alignedduring the arranging the cap wafer and the device wafer.
 13. The methodof claim 12, wherein the cantilever is formed in the device wafer duringthe forming the movable mass.
 14. The method of claim 13, wherein thepost contacts the cantilever during the performing the evacuation of thegas and depresses the cantilever in response to the pressing the capwafer and device wafer together.
 15. A multi-wafer structure having acap wafer and a device wafer, comprising: a cavity in a die of the capwafer; a movable mass in a die of the device wafer and in the cavity; aeutectic bond contacting the cap wafer and the device wafer and sealingin a low pressure in the cavity; and a collapsed standoff structurearound the cavity and contacting the cap wafer and the device wafer,wherein the collapsed standoff structure comprises one of a groupconsisting of fractured pieces in the collapsed structure and a postdepressing a cantilever adjacent to the eutectic bond.
 16. Themulti-wafer structure of claim 15, wherein the eutectic bond comprisesgermanium and aluminum.
 17. The multi-wafer structure of claim 15,wherein the fractured pieces comprise polysilicon.
 18. The multi-waferstructure of claim 15, wherein the cantilever is of the same material isthe movable mass.
 19. The multi-wafer structure of claim 15, furthercomprising an opening in the device wafer adjacent to the eutectic bondon a side opposite that adjacent to the movable mass.
 20. A method offorming a MEMS device, comprising: forming a cavity in a cap wafer;forming a movable mass in a device wafer; forming a first materialsurrounding the cavity; forming a second material surrounding themovable mass; forming a collapsible standoff; aligning the cap wafer andthe device wafer so that the movable mass is in the cavity and the capwafer and device wafer are held in separation by the collapsiblestandoff; reducing the pressure in the cavity with the aid of applying aheat greater than a eutectic temperature and of the first material andthe second material; reducing the temperature below the eutectictemperature; contacting the first material and the second material witha force sufficient to collapse the collapsible standoff; and applying atemperature greater than the eutectic temperature to form a eutecticbond with the first material and the second material to seal in a lowpressure in the cavity with the movable mass.